1. Field of the Invention
This invention relates to a transmission line switch and, in particular, to a switch for the transmission with gain of a signal from one of a plurality of input lines connected to a common output line.
2. Description of Related Art
For many years the PIN diode has dominated as the control element in switches for microwave circuits. More recently attention has focussed on the use of single- and dual-gate FETs in the design of fast switches. Configurations using FETs include series and shunt mounted arrangements, both relying on the drain-source resistance of the device in the `on` state as a low impedance path, either for transmission of the signal (series configuration), or as a shunt across the line (shunt configuration). Combinations of series and shunt mounted devices are also known, which further improve isolation in the `off` state of the switch. All these arrangements provide a broadband (untuned) response. The insertion loss in the `on` state can be reduced somewhat by the addition of appropriate tuning components. However, a diode or FET used as a switch in this way causes a degree of signal attenuation, a loss which adds to the noise figure of the overall system in which it is a part. Also, none of these configurations makes use of the amplifying capabilities of the FET device. In applications where a transmission line switch is at the front end of a microwave receiving system, where, for instance, the switch may be required to select one of two DBS (Direct Broadcast by Satellite), also known as satellite TV, broadcast signals having different polarizations, the performance of the switch in terms of noise figure, frequency response and isolation will have a profound effect on the quality of the signal available to the rest of the system. In such a case, the use of a FET device as a switch providing gain has the significant advantage that the noise figure of the switch, which, being at the front end of the receiving system, is the most significant stage in terms of noise performance, is substantially that of the amplifying circuit.